Scientists create first-ever ‘movie’ of traveling electric charges inside solar cell

Researchers at the University of California Santa Barbara (UCSB) used the scanning ultrafast electron microscope (SUEM) to capture the first-ever images of electric charges moving across semiconductor materials inside a solar cell. The ability to see the charges in action will help ascertain theories and indirect measurements made in semiconductor materials, a university press release said.

Science textbooks and internet pages are filled with the theory about how semiconductor materials behave and how charges are carried within them. This theory is used in a variety of applications, from solar cells to computer chips.

However, these applications have a common complaint—the generation of excess heat. Because the device is often operated continuously, this heat is often let out. Whether powered by electricity or the sun’s rays, this is a waste of energy, which could be improved to make devices more energy efficient.

The clues to doing so are likely hiding in the photocarriers.

Watching photocarriers

The workings of a solar cell are commonly explained as follows: Sunlight hits the semiconductor material and excites the electrons, causing them to move. This movement creates a current, separating them from their oppositely charged ‘holes.’ The current is harvested by the solar cell apparatus, and the electrons go back to their holes.

The excited electrons or photocarriers lose their energy within picoseconds (10^-12), and the solar cell only captures a small fraction of the energy. The rest is released as heat. If photocarriers are caught earlier in their ‘hot’ state, more energy can be harvested from them.

However, semiconductor-based applications are a bit more complex and often use multiple materials. The electrons have to move across their interface, which is referred to as the heterojunction. Visualizing the photocarriers across heterojunctions is tricky, and that’s why the research team led by Bolin Liao, an associate professor of mechanical engineering at UCSB, turned to ultrafast electron microscopy.

The picosecond scale shutter

“If you excite charges in the uniform silicon or germanium regions, the hot carriers move very, very fast; they have a very high speed initially because of their high temperature,” explained Liao in a press release. “But if you excite a charge near the junction, a fraction of the carriers are actually trapped by the junction potential, which slows them down.”

Liao and his team used a heterojunction of silicon and germanium in their research because it has potential applications in solar panels and telecommunications. Since the movement of the photocarriers occurs in a matter of picoseconds, the researchers had to create a picosecond-scale shutter to capture the image of the charges moving.

The team used ultrafast laser pulses to fire electron beams at the heterojunction to visualize the movement activated by an optical beam. “What we’re talking about are events happening within this picosecond to nanosecond time window,” added Liao. “Basically, we’re trying to add time resolution to electron microscopes.”

The researchers were successful in visualizing what semiconductor theory explains. More importantly, it also demonstrates the ability of electron microscopy to study semiconductor devices.

The research findings were published in the journal PNAS .